Method of seasoning pyroxylin plastic, particularly sheets



Patented May 8, 1934 UNETEQ STATES PAT QFFICE George Charles Gress, Springfield, Mass, as-

signor to The Fiberloid Corporation, Indian ()rchard, Mass a corporation of Massachusetts No Drawing. Application July 29, 1931,

Serial No. 553,843

3 Claims.

My invention relates to a new and improved method of seasoning pyroxylin plastic, particularly sheets, that is, removing from the pyroxylin plastic 9. sufficient part of the contained solvent 5 to bring the residual solvent down to the proportion ordinarily permitted in so-called pyroxylin plastic seasoned sheets. By solvent I mean a liquid capable of forming with the other ingredients of the mix a colloidal solution. Py-

roxylin plastic sheets where first formed, for instance as they come from the sheeter, contain from 9 to 18 per cent of solvent, which must be reduced to about 4.25 per cent or less for sheets .350 inch in thickness and down to 0.50 per cent or less for sheets of .020 inch in thickness.

As is well known to those skilled in the art, the process of removing the necessary part of the solvent and thereby seasoning the pyroxylin plastic is long and expensive. At the present time 20 and using so-called laminated glass stock as an illustration which is about .020 inch thick, it is ordinarily considered necessary to expose the sheets after they have been produced by the sheeter to a carefully regulated atmosphere for a period of from ten days to two weeks or even longer. It will be understood, of course, that owin to the long seasoning period the amount of stock in the process is enormous; that the dryer capacity for this large amount of stock must be correspondingly large; and that the work of handling the sheets is expensive. Since the sheets, when hung in the dryers, are separated from each other by vertical passages filled with air charged with inflammable solvent, they are 5 ideally arranged for the propagation of fire, and

since the currents of air pass the sh ets may produce charges of static electricity, the fire hazard is very great.

I am aware that heretofore it has been pro- 49 posed to remove a part at least of the contained solvent by immersion of the sheet in another liquid which is itself mixable with the solvent. For instance, if the contained solvent is alcohol, it may be removed by immersing the sheet in a suitable naphtha. This process, however, is extreme- 1y dangerous owing to the inflammable character both of the nitrocellulose and the naphtha. It has also been known that the immersion in water will remove the contained solvent. However, it has not been considered practical to use water because water renders the pyroXylin plastic sheet cloudy or translucent and the stock itself somewhat brittle or crumbly and reduces its tensile strength and elasticity. If this process is to be used for transparent sheets it is necessary to vaporize or fume the sheets afterwards to render them sufficiently transparent for inspection purposes.

My present invention is based on the discovery that pyroxylin plastic containing a considerable proportion of solvent may be seasoned, by the use of water without clouding the surface or changing the character of the stock provided the seasoning the air is first carried far enough to set the stock. In this case the action of the water not only does not have any deleterious effect either on the surface of the stock or on its structure, but on the contrary produces a smooth glass-like transparent surface on the stock which is not equalled by any method of seasoning known to me.

For purposes of illustration the following description of one method of practicing my invention is given: In this case the sheets are made by the ordinary cake process, that is, they are sliced from a block or cake of pyroxylin plastic containing ordinarily from 9 to 18 per cent of solvent which is usually alcohol. The sheets are then placed between sheets of cardboard in the ordinary way and allowed to remain from 3 to 6 hours. This reduces the amount of contained solvent considerably-usually down to from 8 to 12 per cent of residual solvent, and hardens the surface of the sheet surficiently so that it will not be injured during the later steps of the process. Next the sheets are piled between sheets of dry cardboard in hot air dryers at a temperature of from EEO-100 F. and allowed to remain the balance of twenty-four hours, i. e. from 18 to 21 hours. This air drying treatment has the efiect of hardening the surface of the pyroxylin plastic sheet which is set in its normal transparent condition. The solvent is reduced to about 3-5 per cent for stock of .020 inch thickness. The sheets are then removed from the cardboard and are then placed in a bath of hot water. This bath may be of any temperature up to the boiling point of the solvent. The maximum for alcohol and camphor appears to be about 165 F. but I find that 150 is a satisfactory temperature. I have found that the rate at which the solvent is extracted from the sheet depends on the temperature of the water and that the hotter the water the higher the rate of extraction. The sheets are kept in this bath and the water maintained at the desired temperature mentioned for a length of time depending on the temperature of the water, the amount of contained solvent in the stock, and the amount to be extracted. I find that for stock which is 0.050 inches thick and the water at 150 F. the residual solvent can be reduced to 1.9 per cent which is a satisfactory amount for this thickness of stock. With water at the same temperature, and stock 0.020 inches thick the residual solvent can be reduced to below 0.50 per cent in about forty hours. After the requisite period in the bath, the sheets are removed and the water dried out. This can be accomplished by wiping them or by hanging them while wet in a suitable drier. In neither case are water marks produced on the'surface. In practice and to insure good results I think that the contained solvent should be reduced to about 3-5 per cent for stock of .020 inch thickness before the sheet is placed in the bath although the exact point for different kinds of stock cannot be definitely stated, but depends upon the thickness of the sheet stock and on the character of the material.

Sheets treated as described have a surface which is clear, transparent and smooth and do not require to be vaporized for the particularly rigid inspection used in the manufacture of non-shatterable glass and are equal if not superior in all respects to stock produced by the ordinary cake process and seasoned 14 days and subsequently vaporized. It will, therefore, be seen that my process reduces the time of seasoning from a minimum of ten days and a maximum of fourteen days by the old process to a minimum of two days and a maximum of three days by my new process. The stock in process and the storage space required are proportionately reduced. The expense for labor and handling is ordinarily less and the fire hazard since the stock is immersed in water is practically none. I also find that stock which has been seasoned by my novel process herein described is much freer from strains and stresses than stock made by the old process, and therefore lies flatter and has less tendency to curl. Its surface is not likely to be contaminated by dust particles since it is not exposed for long periods of time to currents of air the friction of which produces static attraction. The solvent loss is lower since solvent which goes off in the air is ordinarily lost while that taken up by the water bath may be recovered. Its unclouded transparent surface makes it unnecessary to vaporize the stock for inspection.

While I am uncertain as to the exact nature of what takes place, I think it probable that in the preliminary air drying the rapid escape of the volatile solvent from the surface fixes the physical structure to such an extent, that the subsequent immersion in water does not affect the surface, although the residual solvent may still be extracted from the sheet by contact with the water. This will perhaps be better understood when it is remembered that ordinary air dried pyroxylin plastic such as a sheet or a tooth brush handle is wholly unaffected by water at room temperatures, while such a sheet or article which has just been formed and therefore contains perhaps 10 per cent or more of solvent, will be rendered cloudy and Weak by immersion in water. In my process the preliminary seasoning is merely sufficient to prevent the change of physical structure which would otherwise occur.

I claim:

1. The process of seasoning sheet pyroxylin plastic which consists in forming the pyroxylin plastic material into a sheet while still containing the original solvent, removing a portion of the solvent by exposure to the air until the surface of the sheet is set in its normal unclouded condition and then removing more of the solvent by immersing the sheet in water.

2. The process of seasoning sheet pyroxylin plastic which consists in forming the pyroxylin plastic material into a sheet while still containing the original solvent, removing a portion of the solvent by exposure to the air until the surface of the sheet is set in its normal unclouded condi+ tion and then removing more of the solvent by immersing the sheet in hot water.

3. The process of seasoning sheet pyroxylin plastic which consists in forming the pyroxylin plastic material into a sheet while still containing the original solvent, removing a portion of the solvent by exposure to the air until the surface of the sheet is set in its normal unclouded condition and then removing more of the solvent by immersing the sheet in water at a temperature of about 150 F.

GEORGE CHARLES GRESS. 

